Both synchrotron radiation photoemission spectroscopy (PES) and Al Kα photoelectron spectroscopy (XPS) are used to determine the suboxide distribution at SiO2/Si (100) interfaces. High resolution PES measurements clearly resolved various suboxides with chemical shifts of 0.97, 1.80, and 2.60 eV for Si+1, Si+2, and Si+3, respectively. A total of 9.3×1014 atoms cm−2 of suboxide is found by PES measurements while only 4.2×1014 atoms cm−2 is measured by XPS on the same sample. This discrepancy is neither caused, as previously believed, by a difference in SiO2/Si (100) quality nor by a difference in methodology in data analysis. The possible factors, e.g., electron mean‐free path and photoionization cross section, which contribute to the difference between PES and XPS data, are considered.